Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions. In general, capacitive sensors are intended to replace mechanical buttons, knobs, and other similar mechanical user interface controls. A capacitive sensor permits eliminating complicated mechanical switches and buttons, providing reliable operation under harsh conditions. Capacitive sensors are widely used in the modern consumer applications, providing new user interface options in existing products (cell phones, digital music players, personal digital assistances, etc.).
One class of capacitive sensor uses a charge transfer technique to sense the capacitance of a sensing capacitor. In one example, the sensing capacitor is first charged using a supply voltage. The charge accumulated on the sensing capacitor is then transferred to an integrating capacitor. The stages of charging the sensing capacitor and transferring the charge to an integrating capacitor are performed repeatedly in response to a first clock source such that a voltage on the integrating capacitor ramps upwards with respect to time. The voltage on the integrating capacitor is then compared to a predetermined reference voltage. The time that it takes the voltage on the integrating capacitor to exceed the reference voltage is related to the capacitance of the sensing capacitor. Thus, the capacitive sensor may also include a second clock source and additional circuitry to measure the amount of time that it takes the voltage on the integrating capacitor to exceed the reference voltage. This measured time may then be used to determine the capacitance of the sensing capacitor.
The above-described capacitance sensor functions properly if the supply voltage and reference voltage do not change. However, changes in temperature or humidity of the capacitance sensor and/or supporting circuitry will often cause one or more of the supply voltages and/or reference voltages to drift. Also, changes in line voltage may cause the supply voltage to change (e.g. voltage spike, externally coupled noise, etc.).
A change in the supply voltage will cause the voltage on the integrating capacitor to charge faster or slower depending on the change in the supply voltage. A change in the reference voltage will cause the voltage on the integrating capacitor to exceed this reference either earlier or later due to the changed reference. Either way, the measured amount of time that it takes the voltage on the integrating capacitor to exceed the reference voltage will change in response to a change in the supply and/or reference voltage resulting in inaccuracies of the measured capacitance.